Electron lens



Mmh 24, 1942. s. RAMO 2,277,414

ELEGTRN LENS Filed July 2, 1941 2 sheets-sheet 1 lInventor:

n Simon 5320,

.Wm/1,76. M4 by His Attorney March 24, 1942. s. RAMO ELEC'IROI(v LENS Filed July 2, 1941 2' @sheets-sheet 2 Inventor: Simonn, b V d y Attorney Patented Mar. 24, 1942 ELECTRON LENS ASimon Ramo, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 2, 1941, Serial No. 400,809

(Cl. Z50- 162) 7 Claims.

The present invention relates to an improved electron lens.

It is known that the component rays of an electron beam can be focused by the action of apertured electrodes spaced along the beam path and supplied with suitable potentials, this combination being appropriately designated an electron lens. 'I'he focusing produced by such a lens is a function of the strength and form of the electrostatic elds existing between the various lens electrodes and is strinkingly analogous to the focusing of a light beam by an optical lens. It has been usefully employed in one instance in the so-called electron microscope, which is an apparatus for obtaining a greatly enlarged electron-optical image of a minute object desired to be investigated.

Like uncorrected optical lenses, electron lenses as heretofore employed are subject to spherical aberration. In both types of lens, the presence of spherical aberration leads either to blurring of the resultant image or to the enforced use of a construction in which only a limited portion of the lens area (that nearest the lens axis) is usefully employed.

It is an object of the present invention to provide an electron lens construction in which spherical aberration is minimized or substantially avoided.

In this connection an important feature of the invention consists in a three-electrode construction in which the central electrode is provided with a generally cylindrical opening and in which each of the other two electrodes presents to an extremity of the said opening a convex surface of revolution. In the preferred case, the generatrix of the convex surface so presented is defined by a particular relationship between the principal dimensional parameters of the lens, this relationship being set forth and explained in the following.

The features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. l is a longitudinal sectional View of an electron microscope suitably embodying the invention, Fig. 2 is a diagrammatic representation useful in explaining the invention, Figs. 3 and 4 illustrate varying electrode configurations within the scope of the invention, Fig. 5 depicts a practical method of forming a tainer is closed by a glass window II having a.

fluorescent material I2 on its inner surface, and at the other end of the container there is provided a glass insulator I4, which serves to support an electron source in the form of a cathode I5. The cathode I5 is surrounded by a tubular metal member I6 which confines the emitted electrons to a narrow beam and is cooperatively positioned with respect to an apertured electrode I8 which is in contact with the main envelope part IIJ. In the normal use of the apparatus the envelope I8 and the apertured electrode I8 are maintained at ground potential andthe cathode is maintained at a high negative potential (e. g. by connection to a potential source 20) so that electrons emitted from the cathode are projected toward the uorescent screen I2.

In using the apparatus as an electron microscope it is desired to cause the electron stream proceeding from the cathode I5 to produce in the plane of the screen I2 an enlarged electronoptical image of a minute object to be investigated. To this end there is provided a suitable means for supporting an object of the type in question in the path of the electron stream, such means being illustrated in the present case as a metal diaphragm 22 provided with a central opening 23 and having a fine mesh screen 24 covering this opening. The object to be investigated (not shown) is applied to the screen 24 in a region overlying a screen opening which is traversed by the longitudinal axis of the microscope. The introduction of the object is accomplished through a suitable vacuum-lock (not shown) provided in a wall of the microscope, and the microscope is thereafter evacuated by connection to an appropriate pumping system.

Between the object support v22 and the selected imaging plane (i. e. the screen I2) there is provided an electron lens for exerting refractive forces on the electron rays proceeding fromthe object. According to the present invention this lens comprises a series of three electrodes 25, 26 and 2l, these being of a particular configuration determined in a manner to be set forth in the following.

The electrodes 25 and 2l are of substantially identical form and are both supported directly from the main envelope part Ill so as to be at the same potential as that part. The central electrode 26 is insulatingly supported by the combination of a heavy conductor 29 and a vitreous insulating cylinder 3U. A metallic disk 3| which is peripherally sealed to the cylinder 30 and which is centrally joined to the conductor 29 provdes an accessible terminal for connecting the electrode 26 to the potential source 20. By establishing a potential difference between the electrode 26 and the electrodes 25 and 21, a lens field may be set up in the interelectrode space of such character as to focus a magnified image of the object under investigation on the screen I2.

It is highly desirable that the image thus produced be free 4from blurring due to spherical aberration of the lens system, and it will be shown in the following that this can be accomplished by proper shaping of the lens electrodes. It has been demonstrated in a paper by Frank Gray published in the Bell System Technical Journal for January, 1939, pages 1 to 31, that the spherical aberration characteristics of an electron lens may be expressed by the relationshipz of the given electron as measured along the lens axis and depends on the starting conditions of the electron and the lens potential distribution; V expresses the potential distribution along the lens axis and is thus obviously a function of z; 1' is the radial displacement of the given electron from the lens axis, and B and C are quantities similar to A and of relative magnitude determined by the design of the lens system in question.

The avoidance of spherical aberration is accomplished if all electrons which start from a particular point on the lens axis have the same focal distance when they pass through a given transaxial lens plane, irrespective of the 'radial displacement of the various electrons at the instant of such traversal. This obviously means that in such an aberration-free lens, focal distance must be independent of radial displacement from the lens axis or, in other words, that the terms Brz, GT4, etc. in Equation 1, above, must vanish.

In a system such as that under consideration the radial displacement 1' is necessarily very small, so that the Cr4 term and all terms containing higher powers of r may be neglected. The conclusion is therefore indicated that if the coefficient B in Equation 1 can be made zero, aberration will be avoided, at least to a first approximation. I

The publication of Gray above referred to shows that a sufficient condition for B to be zero is that indicated by the following differential equation:

in which V is the potential distribution along the lens axis (z).

A solution of this equation which is relevant to the present invention is as follows:

V=F cosh we J0(wr) (3) where Jo indicates a Bessel function, and F and w are arbitrary constants. It will now be shown that this equation may be satisfied by the use of a lens comprising a central electrode of cylindrical character and a pair of complementary electrodes each of which presents to one extremity of the central electrode a convex surface of revolution whose generatrix is determined (in the ideal case) by the following equation:

1 as o2,

where is potential at any point. One solution of this equation is:

where K1 and K2 are perfectly arbitrary constants.

Let K1 be set equal to cosh 2.4 a

and K2 equal to 2.4/a where E (i e., the potential of 26'), b and a are quantities identified in Fig. 2. Then Equation 6 becomes i a T cosh 2.45

which is an equation precisely in the form of Equation 3 specified above as determinative of the condition of minimum first order spherical aberration of the lens in which the arbitrary constant F takes on the particular value cosha JU (7) cosh 24g U,

and the constant w=2.4/a. Moreover, Equation '7 complies with the condition that the geometry of the end electrodes 25 and 26 shall be determined by Equation 4, according to which 2.4 2.4 cosh 2.47;=cosh -a-aJU r) For when the latter equation is satised (which is true only on the surface of the end electrodes), the potential o of Equation 7 becomes cosh 2.4i 2 4 f): E (s) which obviously accords with the situation illustrated in Fig. 2. Also, when r equals a (which is true at the surface of the 'central lelectrode 26') and from (7) which correctly describes `the postulated potential condition of electrode 26.

The electrode lshape defined by Equation 4 obviously varies with the Adimensional parameters a and b, and `in Figs. 3 and 4 there are 4shown a series of configurations which result when different dimensions fare chosen. Figure 3 illustrates particularly the effect of `changing the diameter a of the central electrode while leaving the spacing `b of the end electrodes fixed, and Fig. 4 il- `lustrates the result of changing the spacing b while maintaining a xed diameter of the central electrode.

It will `be apparent from what has been said in the foregoing that the `ideal case from the standpoint-of avoiding aberration is realized when the end electrodes correspond exactly to the configuration defined by Equation :4. It -is found, however, that in many instances an `adequate approximation may be obtainedby the use of a convex surface of revolution of generally spherical character, the radius of curvature of the surface being chosen to cause the surface to conform as nearly as possible to the ideal shape. Where a still closer approximation is desired, this may be obtained by the use of an electrode formed with two merging spherical surfaces of different radii of curvature as indicated in Fig. 5. (In this latter ligure the two merging spheres are designated as .r and y and the solid line u depicts the resultant electrode structure.)

In view of the foregoing it will be understood that the advantages of the present invention may be realized to a substantial degree not only by the exactly formed structures described by Equation 4 but also by the use of convex end electrodes whose surfaces are quasi-spherical (including spherical) sections formed to approximate but not necessarily to coincide with the ideal case. Accordingly, it is considered that constructions of the latter character are Within the generic scope of the invention.

It is, of course, unnecessary that the electrode structures be of the precise character illustrated in Fig. 1, and in Fig. 6 there is shown a construction which may be alternatively employed. In this case the central electrode 40 is in the form of a thick circular plate having a central opening 4| of generally cylindrical form. This is positioned between two additional plate-like members 43 and 44 which act as complementary lens electrodes and which are assumed to be maintained at a common potential difference with respect to the electrode 40. In order to minimize spherical aberration effects the electrodes 43 and 44 are provided centrally with protuberances 45 and 46 in the form of quasi-spherical sections which face the respective extremities of the opening 4i. Each of the electrodes is provided with a small central aperture (numbered 48 and 49 respectively) which is in alignment with the opening 4| and which serves to permit the passage of an electron stream through the lens system.

It will be readily apparent that insofar as the lens-forming surfaces are concerned the electrode arrangement of Fig. 6 is the equivalent of that of Fig, 1. That is to say, assuming the prot-,uberant surfaces 45=and46 Yto .be appropriately ,shaped with Lreference to the spacing `of ytheir lrespective electrodes and the diameter of the opening 4i (i. e. in accordance with Equation 4, above), .the lens action occurring in the interelectrode space will be -free or 4substantially free Vof spherical aberration.

In most constructions, it will prove desirable to arrange the various electrodes so that at least a portion ofthe convex surface of each of the end electrodes extends within the opening of the central electrode. Accordingly,v this 4arrangement may be considered a preferred, although not essential embodiment of the invention.

While the invention has been described mainly by vreference to particular structures and to a particular application, it will be understood that numerous modifications both of construction and use will occur to vthose skilled in the art. I, therefore, aim inthe appended claims to cover all such equivalent variations `as come within the true spirit vand scope of the foregoing disclosure.

What I claim `as new and desire Vto secure by Letters Patent of the United States is:

l. An electron lens comprising a first electrode having a generally cylindrical opening therethrough and a'pair of `complementary electrodes on opposite sides of the first electrode and having relatively small openings in alignment with the opening in said first electrode, said complementary electrodes each presenting to said first electrode a convex surface of revolution in the form of a quasi-spherical section which is symmetrical about a line directed through said openings, whereby spherical aberration of said lens is minimized.

2. An electron lens comprising a first electrode having an opening through it of generally cylindrical form and a pair of complementary electrodes on opposite sides of the first electrode and each presenting to it a convex surface of revolution in the form of a quasi-spherical section having a small aperture in axial alignment with the said opening, the said complementary electrodes being adapted to be maintained at a common potential difference with respect to the Afirst electrode during normal use of the lens, whereby the lens eld produced by the electrodes is symmetrical with reference to the central transverse plane of the said opening.

3. An electron lens comprising a cylindrical tubular electrode and a pair of complementary electrodes adjacent to the respective extremities of the tubular electrode and adapted to be maintained at a common difference of potential with respect to it, the said complementary electrodes each having an aperture aligned with the axisof the tubular electrode and each presenting to said electrode a convex surface of revolution in the form of a quasi-spherical section which is symmetrical about said axis.

4. An electron lens comprising a first electrode having an opening through it of generally cylindrical form and a pair of complementary electrodes on opposite sides of said electrode and each presenting a convex surface of revolution which extends partially within the said opening, said complementary electrodes having relatively small openings which are respectively coaxially aligned with the opening in said first electrode, and the said convex surfaces being symmetrical about the common axis of the various openings.

5. An electron lens comprising a first electrode having through it a generally cylindrical opening and a pair of complementaryelectrodes on opposite sides of the rst electrode and each having a relatively small aperture in alignment with the said opening, each of the said complementary electrodes presenting to the rst electrode a convex surface of revolution the generatrix of which is dened at least approximately by the equation:

where a is the radius of the said opening, bis one half the spacing of the complementary electrodes, and r and a are variable distances measured respectively from the longitudinal axis and from the central transverse plane of the lens.

6. In electronic apparatus for producing in a selected plane an electron-optical image of an object desired to be investigated, the combination which includes an electron source, means for supporting the object to be investigated in the path of electrons projected from said source, and an electron lens positioned between the said object-supporting means and the selected imaging plane, said electron lens comprising a first electrode having through it a generally cylindrical opening adapted to be traversed by electrons proceeding from the object-supporting means and a pair of further electrodes eacfi presenting to an extremity of the said opening a ccilvex surface of revolution in the form of a quasi-spherical section and having a relatively small aperture in alignment with the opening.

7. In electronic apparatus for producing in a selected plane an electron-optical image of an object desired to be investigated, the combination which includes an electron source, means supporting the object to be investigated in the path of electrons projected from the said source, and an electron lens positioned between the said object-supporting means and the selected imaging plane, said electron lens comprising a central electrode having through it a generally cylindrical opening adapted to be traversed by electrons proceeding from the object supporting means, a pair of further electrodes each presenting to an extremity of the said opening a convex surface of revolution and having a relatively small aperture in alignment with the opening, and connections for maintaining said further electrodes at a common potential difference with respect to the central electrode, whereby the lens field produced by the electrodes during normal use thereof is symmetrical with reference to the central transverse plane of the lens.

` SIMON RAMO. 

